1. Field of the Invention
The present invention relates to a method of preparing a phosphor layer coating solution of a radiation image conversion panel used for a radiation image conversion method which utilizes a stimulable phosphor, and to a method of manufacturing the radiation image conversion panel.
2. Description of the Related Art
As a method used in place of a conventional radiography, there has been known a radiation image conversion method which uses a stimulable phosphor, for example, as disclosed in Japanese Patent Application Laid-Open (JP-A) No. 55-12145. This method uses a radiation image conversion panel containing a stimulable phosphor (which may also be referred to as an accumulative phosphor sheet). In this method, a stimulable phosphor of a radiation image conversion panel absorbs radiation transmitted through a subject or emitted from an object to be examined. Thereafter, the stimulable phosphor is excited over time by an electromagnetic wave (excited light) such as a visible ray or an infrared ray to thereby allow the radiation energy stored in the stimulable phosphor to be released as fluorescence (stimulable emission). An electric signal is obtained by photoelectrically reading the fluorescence, and the radiation image of the subject or the object to be examined is produced as a visible image on the basis of the obtained electric signal. On the other hand, after the remaining image on the radiation image conversion panel whose reading has been finished is erased, the radiation image conversion panel is ready for the next photographing. Namely, the radiation image conversion panel is repeatedly used in the above-described manner.
In accordance with the radiation image conversion method, as compared with a case of using radiography in which a combination of a conventional radiographic film and sensitizing paper is used, there is an advantage in that it is possible to obtain a radiation image having a large amount of information with even less exposure (radiation) dose. Further, in the conventional radiography, the radiographic film is used up each time photographing is carried out. In contrast, in the radiation image conversion method, because the radiation image conversion panel is repeatedly used, it is beneficial from the viewpoints of conservation of resources and economic efficiency.
A radiation image conversion panel used in the radiation image conversion method is formed by, as a fundamental structure, a support and a phosphor layer which is provided on a surface of the support and contains a stimulable phosphor. The phosphor layer is generally formed by the stimulable phosphor and a binder which contains and supports the stimulable phosphor in a dispersed state. The stimulable phosphor has a property of exhibiting a stimulable luminescence when the phosphor absorbs radiation such as X-rays and is then irradiated by excited light. Therefore, the radiation transmitted through a subject or emitted from an object to be examined is absorbed in the phosphor layer of the radiation image conversion panel in proportion to the radiation dose (amount of emitted or transmitted radiation). A radiation image of the subject or the object to be examined is formed on the panel as an image storing radiation energy. The image storing radiation energy can be emitted as stimulable emission by irradiating excited light thereon, and the image storing radiation energy can be imaged by photoelectrically reading the stimulable emission and converting it into an electric signal.
The phosphor layer is formed by preparing a phosphor layer coating solution which is prepared by adding a stimulable phosphor and a binder to a suitable solvent, and dispersing and dissolving them, and then applying the phosphor layer coating solution to the surface of the support and drying it. In the preparation of the phosphor layer coating solution, a cross-linking agent is added in view of enhancing the mechanical strength of the obtained phosphor layer. For example, JP-A Nos. 4-315100 and 8-36099 disclose examples in which a polyurethane resin is used as a binder and an isocyanate is used as a cross-linking agent.
However, when a solution, which contains a stimulable phosphor, binder and a cross-linking agent having the action of cross-linking a binder by being heated, such as isocyanate, is stirred in order to disperse the stimulable phosphor sufficiently, the temperature of the solution is raised by the stirring such that a cross-linking reaction of the cross-linking agent is partially caused, and the stimulable phosphor is not uniformly dispersed and may aggregate. Further, there may be cases in which aggregation of the binder itself is caused by the cross-linking reaction of the cross-linking agent.
When the phosphor layer is formed by using a phosphor layer coating solution in which aggregation is caused, defects at the surface due to the aggregation of the stimulable phosphor tend to be caused and the image quality may be lowered due to deterioration of structural mottle.
Cooling the entire stirring container by cooling water or the like has been considered in order to prevent the temperature of the solution from being raised. In this case, the cooling effect of the cooling water affects only a portion of the solution contacting a wall portion of the stirring container such that the temperature of the entire solution within the stirring container cannot be kept constant. Accordingly, within the stirring container, the temperature of small portions of the solution subjected to stress due to stirring is raised. Water used for cooling water is usually well water. Although the temperature of well water itself is lower than an ordinary temperature, because cooling is carried out only from the outside of the stirring container, the cooling efficiency is not sufficient to keep the temperature of the solution within the stirring container constant. As a result, because the stirring for dispersing the phosphor is carried out for a long time, the temperature of the solution is raised. As described above, a rise in the temperature of the solution cannot be avoided merely by cooling the whole stirring container by cooling water or the like, and aggregation due to the cross-linking reaction by the cross-linking agent cannot be prevented. Further, unpreferable aggregation of stimulable phosphor cannot be prevented.
An object of the present invention is to provide a method of preparing a phosphor layer coating solution which enables manufacture of a radiation image conversion panel which has a superior condition and superior structural mottle, while enabling enhancement of mechanical strength due to the addition of a cross-linking agent, and a method of manufacturing a radiation image conversion panel.
The above-described object is accomplished by the present invention. A first aspect of the present invention is a method of preparing a phosphor layer coating solution for a radiation image conversion panel, comprising the steps of: (a) providing at least a solvent, a phosphor, a binder, and a cross-linking agent which cross-links the binder by being heated, for constituent components; (b) preparing a phosphor dispersion by mixing and stirring the constituent components, except for the cross-linking agent, to disperse the phosphor; (c) cooling the phosphor dispersion; and (d) adding the cross-linking agent to the phosphor dispersion and stirring until a substantially uniform mixture is obtained.
A second aspect of the present invention is a method of manufacturing a radiation image conversion panel comprising the steps of: (a) preparing a phosphor dispersion by mixing and stirring at least a solvent, a phosphor and binder together; (b) cooling the phosphor dispersion; (c) preparing a phosphor layer coating solution by adding a cross-linking agent to the phosphor dispersion after the step of cooling, and stirring; and (d) forming a phosphor layer by applying the phosphor layer coating solution across a surface of a support.
In the present invention, drawbacks, which are caused by a phosphor layer coating solution which contains a cross-linking agent having the action of cross-linking a binder by being heated, are solved by providing the cross-linking agent adding step after the phosphor dispersing step.
Namely, the phosphor is sufficiently stirred and dispersed in a liquid state in which the cross-linking agent is not contained (phosphor dispersing step). Then, the phosphor solution which generates heat is cooled (cooling step) and the cross-linking agent is added thereto (cross-linking agent adding step). The phosphor dispersing step is separated from the cross-linking agent adding step such that the phosphor dispersing step and the cross-linking agent adding step can be respectively carried out at optimum conditions. As a result, a phosphor layer coating solution can be prepared without any aggregation being caused. In this way, since a radiation image conversion panel is formed by using the phosphor layer coating solution which does not contain aggregates and in which the phosphor is sufficiently dispersed, an extremely excellent surface condition of the phosphor can be obtained and superior structural mottle can also be provided. Further, the effect of enhancing the mechanical strength due to the addition of the cross-linking agent can also be exhibited.